Store is a general-purpose concurrently-accessible copy-on-write store built
on top of the github.com/steveyen/gtreap treap (tree + heap) implementation.

A Store is composed of a series of named Collection instances, each of which
can hold data elements.

A Store is created by calling New, and is initially in a read/write mode.
Derivative Stores can be created by calling a Store's Snapshot capability.
These Stores will be read-only, meaning their collections and those
collections' data data may only be accessed, not modified.

A Store's zero value is a valid read-only empty Store, but is not terribly
useful.